During PHI elimination, split critical edges that move copies out of loops

[djasper]

Summary

This prevents the behavior observed in llvm.org/PR22369. I am not sure whether I am reading the code correctly, but the early exit based on isLiveOutPastPHIs() seems like a bug as it prevents the nice loop-splitting logic below.

This is still work in progress as it currently breaks a few tests. Mainly, it seems to influence which copies are local and which are non-local, which influences the order in which the RegisterCoalescer visits copies. Coalescing the wrong registers first has unwanted side-effects.

[qcolombet]

Hi Daniel,

This is still work in progress as it currently breaks a few tests. Mainly, it seems to influence which copies are local and which are non-local, which influences the order in which the RegisterCoalescer visits copies. Coalescing the wrong registers first has unwanted side-effects.

What does this mean for the livability of this patch?

Thanks,
-Quentin

[djasper]

Not sure what that means. I think the changes here by itself are good, but they lead to worse behavior in later passes, specifically register coalescing. I'll take a look at whether the register coalescer heuristic can be tweaked in a simple way to avoid this regression.

[qcolombet]

My concern was this:

it currently breaks a few tests

What do you mean by "break"?

From your last comment, I believe we generate worse code.
If that the case, then just put a flag to preserve the previous behavior and file PR for the regression you find so that we can work on fixing them.
Other than that, the change itself looks good to me.

Thanks,
-Quentin

[djasper]

• Hide new behavior behind flag.
• Add test.

[djasper]

I have hidden the new behavior behind a flag and added a test.

With the flag enabled by default, these four tests break:

LLVM :: CodeGen/Hexagon/hwloop-cleanup.ll

triggers an assertion.

LLVM :: CodeGen/X86/coalescer-commute4.ll
LLVM :: CodeGen/X86/phys_subreg_coalesce-2.ll
LLVM :: CodeGen/X86/zlib-longest-match.ll

generate worse code. Likely these need to be solved by improving the register allocator.

[MatzeB]

The change itself LGTM (but nitpick below).

As mentioned in by you on IRC we perform some unlucky choices in the coalescing order in test/CodeGen/X86/coalescer-commute4.ll with this patch. Are there more? If it's just some tests like this failing but benchmarks generally improving, then it's okay to XFAIL the tests, or trying to rewrite them in a way that we are lucky with the heuristic. If benchmarks generally regress with the changes, then we need further research on how to avoid that...

lib/CodeGen/PHIElimination.cpp

576–577

No need to keep the old code in a comment, it can always be found in the subversion log.

[MatzeB]

In D8016#133116, @MatzeB wrote:

The change itself LGTM (but nitpick below).

As mentioned in by you on IRC we perform some unlucky choices in the coalescing order in test/CodeGen/X86/coalescer-commute4.ll with this patch. Are there more? If it's just some tests like this failing but benchmarks generally improving, then it's okay to XFAIL the tests, or trying to rewrite them in a way that we are lucky with the heuristic. If benchmarks generally regress with the changes, then we need further research on how to avoid that...

You answered to most of my remarks while I was writing this :)

[djasper]

The old code is already removed in the latest version of the patch.

I think the "unlucky" ordering happens in three tests as per my previous comment and it also regresses benchmarks in our internal suite, although I haven't looked closely at which ones. Thus, I have guarded the new behavior by a flag now as Quentin suggested. I'll then try to fix the unlucky behavior and see whether that also improves benchmarks.

[qcolombet]

Thanks Daniel for the flag.

We could switch the default value as soon as we fix the assertion. In the meantime, please file PRs for the regressions so that we can help resolving those.

LGTM.

Cheers,
-Quentin

[djasper]

Submitted as r231064.

Also filed:

llvm.org/PR22767
llvm.org/PR22768

lib/CodeGen/PHIElimination.cpp

for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) {
         continue;
 
       // LV doesn't consider a phi use live-out, so isLiveOut only returns true
       // when the source register is live-out for some other reason than a phi
       // use. That means the copy we will insert in PreMBB won't be a kill, and
       // there is a risk it may not be coalesced away.
       //
       // If the copy would be a kill, there is no need to split the edge.
-      if (!isLiveOutPastPHIs(Reg, PreMBB) && !SplitAllCriticalEdges)
-        continue;
-
+      // if (!isLiveOutPastPHIs(Reg, PreMBB) && !SplitAllCriticalEdges)
+      //   continue;
+      bool ShouldSplit = isLiveOutPastPHIs(Reg, PreMBB);
+      if (ShouldSplit) {
         DEBUG(dbgs() << PrintReg(Reg) << " live-out before critical edge BB#"
                      << PreMBB->getNumber() << " -> BB#" << MBB.getNumber()
                      << ": " << *BBI);
+      }
 
       // If Reg is not live-in to MBB, it means it must be live-in to some
       // other PreMBB successor, and we can avoid the interference by splitting
       // the edge.
       //
       // If Reg *is* live-in to MBB, the interference is inevitable and a copy
       // is likely to be left after coalescing. If we are looking at a loop
       // exiting edge, split it so we won't insert code in the loop, otherwise
       // don't bother.
-      bool ShouldSplit = !isLiveIn(Reg, &MBB) || SplitAllCriticalEdges;
+      ShouldSplit = ShouldSplit && !isLiveIn(Reg, &MBB);
 
       // Check for a loop exiting edge.
       if (!ShouldSplit && CurLoop != PreLoop) {
         DEBUG({
           dbgs() << "Split wouldn't help, maybe avoid loop copies?\n";
           if (PreLoop) dbgs() << "PreLoop: " << *PreLoop;
           if (CurLoop) dbgs() << "CurLoop: " << *CurLoop;
         });
         // This edge could be entering a loop, exiting a loop, or it could be
         // both: Jumping directly form one loop to the header of a sibling
         // loop.
         // Split unless this edge is entering CurLoop from an outer loop.
         ShouldSplit = PreLoop && !PreLoop->contains(CurLoop);
       }
-      if (!ShouldSplit)
+      if (!ShouldSplit && !SplitAllCriticalEdges)
         continue;
       if (!PreMBB->SplitCriticalEdge(&MBB, this)) {
         DEBUG(dbgs() << "Failed to split critical edge.\n");
         continue;
       }
       Changed = true;
       ++NumCriticalEdgesSplit;
     }